Lab 7: RSA Demo

In Lecture 16 you
learned everything you needed to implement RSA key generation in Java using
the fields and methods of the BigInteger class. In your Prelab you wrote the Java code for the
keyGenerate() subroutine. In lab today you will add your
subroutine to the RSA demo applet code and implement the RSA encryption and
decryption subroutines. Once your code is working, you will have an applet
just like this:

Press the Generate RSA Keys button to compute public and private
key pairs. Enter your plaintext message in the box indicated and press
Encrypt. Your encrypted text is shown. Press the Decrypt
button to decrypt the ciphertext. You can generate a new set of keys for
the same message by pressing the Generate RSA Keys button again.
Press Clear to clear all areas of text.

Preliminaries

Download lab7.zip and
extract its contents to the cps001 subdirectory of your
public_html folder. You should now have a folder in your
cps001 directory named lab7. Open the lab7
folder. It should contain two files, RSA.java and
RSA.html.

Starting Emacs: On the desktop of your computer there is a folder
named Class Applications. Open this folder and find the link to
Emacs. Double click on the link to start Emacs.

Open the file RSA.html in Emacs and add your name and today's
date. Save the file.

You should type both your subroutines in the space between these comment
lines. You do not need to modify any other code in the applet.

keyGenerate

Type your prelab code for the keyGenerate subroutine.

crypt

The only thing left to do before you can compile is to add a subroutine
to perform encryption and decryption. In Lecture 16 we discussed how one
subroutine could be used to perform both encryption and decryption, as they
both perform the same computation.

Recall that RSA encyption takes as input a numeric value for the
plaintext message P and the public key pair (k,n). The
numeric ciphertext message C is then computed via the equation

C = (Mk)%n

Similarly, RSA decryption takes as input a numeric value for the
ciphertext message C and the private key pair (d,n) and
decrypts the ciphertext via the equation

D = (Cd)%n

All the values used for encryption and decryption are of type
BigInteger. We can use the modPow method of the
BigInteger class to perform the above computations for us.
Specifically, given BigInteger values x, y and
z, the value of (xy)%z is the
BigInteger value
x.modPow(y,z)

We can write a subroutine crypt to perform both encryption and
decryption as follows. The input parameters to crypt will be
three variables of type BigInteger. The first variable, say
M, represents the numeric text message. The second and third
variables, say pkey and modkey, represent the RSA key
pair for the desired operation - pkey being the unique public or
private key and modkey being the shared modulus key. The
crypt subroutine returns a value of type BigInteger,
which is computed using the modPow method.

Type your code for the crypt subroutine below the code you
just added for keyGenerate.

Compile and test

Compile RSA.java in Emacs by selecting Tools->Compile
from the drop-down menu.

Emacs note: If you want to get back to one window, go to
File->Unsplit Windows on the drop-down menu.

Once your applet has compiled, open RSA.html in a web browser
and test it. If everything runs as it should -- that is, you can generate
keys, enter a message, encrypt it and get your original message back by
decrypting -- move on to the next section and add a link to your applet.